As a speech synthesis method using a statistical method, Hidden Markov Model (HMM) speech synthesis using Hidden Markov Model is known. A prosody generated by HMM speech synthesis is represented by using a specific number of states.
Further, as a method of simultaneously modeling pitch patterns of a voiced sound and an unvoiced sound, MSD-HMM (Multi-Space Probability Distribution HMM) is known (see Non-Patent Literature 1). Upon speech synthesis based on MSD-HMM, to generate a waveform, whether to generate a waveform as a voiced or an unvoiced is determined based on an index (referred to as a voiced utterance likelihood index below) which indicates the degree of voiced utterance likelihood.